Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes. Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could alternatively be a gas.
When a fluid working machine operates as a pump, a low pressure manifold typically acts as a net source of fluid and a high pressure manifold typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold typically acts as a net source of fluid and a low pressure manifold typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” refer to manifolds with higher and lower pressures relative to each other. The pressure difference between the high and low pressure manifolds, and the absolute values of the pressure in the high and low pressure manifolds will depend on the application. A fluid working machine may have more than one low pressure manifold and may have more than one high pressure manifold.
The invention concerns valve assemblies suitable for use in regulating the flow of fluid between a working chamber of a fluid working machine and a manifold. The valve assemblies include a valve member and valve seat and, accordingly, are face-seating. Valve assemblies according to the invention may be useful with a wide range of types of fluid working machine. However, issues concerning the invention will now be discussed with reference to the specific example of valve assemblies suitable for use with known fluid working machines which comprise a plurality of working chambers of cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis and in phased relationship to cycles of working chamber volume, to determine the net throughput of fluid through the machine.
For example, EP 0 361 927 disclosed a method of controlling the net throughput of fluid through a multi-chamber pump by opening and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the pump and a low pressure manifold. As a result, individual chambers are selectable by a controller, on a cycle by cycle basis, to either displace a predetermined fixed volume of fluid or to undergo an idle cycle with no net displacement of fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand. EP 0 494 236 developed this principle and included electronically controllable poppet valves which regulate fluid communication between individual working chambers and a high pressure manifold, thereby facilitating the provision of a fluid working machine functioning as either a pump or a motor in alternative operating modes. EP 1 537 333 introduced the possibility of part cycles, allowing individual cycles of individual working chambers to displace any of a plurality of different volumes of fluid to better match demand.
Fluid working machines of this type require rapidly opening and closing electronically controllable valves capable of regulating the flow of fluid into and out of a working chamber from and into the low pressure manifold, and in some embodiments, the high pressure manifold. The electronically controllable valve which regulates fluid flow between a working chamber and the low pressure manifold is typically oriented such that the valve member (for example, a poppet valve head) moves in the same sense as the direction of fluid flow through the valve seat during an exhaust stroke in which fluid is vented to the low pressure manifold. During an idle exhaust stroke, where fluid which was received from the low pressure manifold in the preceding intake stoke is returned to the low pressure manifold, the peak rate of fluid flow is very high. In these circumstances, forces arising from at least four different phenomena, which we will now describe, urge the poppet valve head towards the valve seat.    1. In order to facilitate rapid fluid flow with minimal energy losses while minimising the distance over which the poppet head must travel, it is advantageous to maximize the cross-sectional area of the fluid flow path, minimise the gap between the valve seat and the poppet head, and maximize the fluid volume around the rest of the poppet head. This means that the fluid flows fastest in the gap between the poppet head and valve seat. As a result, there is a kinetic energy related pressure drop which is at its greatest (leading to a minimum of fluid pressure) between the poppet valve head and the valve seat, which force acts to close the valve. Kinetic energy related forces are proportional to the square of the fluid velocity and fluid density and this force applies during both intake and exhaust strokes.    2. Viscous drag arising from the flow of fluid across the surface of the poppet valve head acts at a tangent to the poppet head surface. There is always a component of the poppet head surface tangent in the closing direction, so viscous drag acts to pull the poppet valve closed during exhaust strokes. Viscous drag is proportional to the product of fluid velocity and fluid viscosity, and is particularly important at low temperatures.    3. As the fluid splits and diverts past the front of the poppet valve head it must internally shear as its shape changes. This leads to a high pressure region on the upstream surface of the poppet valve heat and a low pressure region on the inward surface (where the diverted flow rejoins). The resulting pressure differential acts to close the poppet valve. Shear forces are proportional to the product of fluid velocity and fluid viscosity, and are also particularly important at low temperatures.    4. The fluid mass must also be accelerated perpendicular to the axis of flow so that it diverts around the poppet valve head. The acceleration raises pressure within the fluid at the poppet and therefore also applies forces normal to the face being diverted around (the so-called ‘jet effect’). These forces are felt by the poppet valve head and, because poppet valve heads typically have cone or bullet shaped upstream faces in order to reduce shear forces, the resulting forces have a component in the closing direction of the poppet. These acceleration-related forces are proportional to the product of fluid density and fluid viscosity.
The magnitude of the resulting forces varies depending on temperature, fluid viscosity, instantaneous flow rate and the configuration of the valve. For example, in some valves, fluid impinges on the side of the valve member before flowing between the valve member and the valve seat, rather than the upstream surface, affecting the relative magnitude of these forces.
As a result of these forces, there can be a risk of involuntary pumping strokes occurring, in which fluid is displaced which was not demanded by the controller. This can significantly affect the performance of a fluid-working machine, particularly at low temperature and may be dangerous. It is not practical to overcome these forces simply by providing stronger magnetic fields to hold the valve open as this would increase the power consumption of the valve and slow down release of the valve.
Similar issues may arise in other types of fluid working machine where fluid flows through the valve seat during at least some operating circumstances in the same sense as the valve member moves towards the valve seat to close the valve.
The aim of the invention is to provide a valve assembly suitable for regulating the flow of fluid between a working chamber of a fluid-working machine and a fluid manifold, in which the effects of some or all of these four forces are mitigated or obviated. This is achieved in some embodiments by reducing or removing one or more of these forces and thereby mitigating or obviating the effects of some or all of these four forces.
By mitigating the effects of some of all of these four forces, the likelihood of the valve closing at the wrong time is reduced, and the power consumption of the valve assembly can in some circumstances be reduced. The valve may also have a longer working life than would otherwise be the case.